The present invention relates generally to methods and apparatus for use in casting, particularly to more efficiently producing castings of such items as railroad car connector knuckles.
Casting methods currently used to produce items of metal alloys employ molding techniques that replicate the interior and exterior features of a desired part. Such methods comprise an exterior mold that replicates the external surface features of the desired part, while a core or cores are used to replicate interior cavities and surfaces if such parts embody hollow or reentrant features. The mold and cores are produced from a pattern of the part and are assembled together within containers called “flasks” to produce a cavity that replicates the volume and surface features of the desired part. The mold flask is usually split into two separate components; an upper component traditionally called the “cope” and a lower component called the “drag.” A pattern of the part is placed within the cope and drag over which green molding sand is rammed to replicate the shape of the pattern. The cope and drag are configured to mate with each other to form two halves of the mold cavity to allow the removal of the part pattern from the compacted green sand leaving the desired mold cavity. Cores are subsequently placed within the mold and the mold halves fitted together to form a mold assembly. A system of sprues, runners, gates and risers embodied within the core mold assembly provide the requisite channels to direct molten metal poured into the formed part cavity to reproduce the part. Molten metal is poured into the mold assembly and is allowed to cool and solidify. Once the casting has cooled sufficiently, the cast part is shaken from the sand mold and the cores removed leaving the desired replicated part. The mold and core sand are usually reclaimed and reused.
Of the various types of molding methods used, molds made from “green sand” are the most widely used. Green sand is made from a pliable mixture of sand, clay, and water that coheres and can be molded in such a fashion as to faithfully replicate surface features of the part pattern shape. However, significant disadvantages are associated with the green sand method, some of which are the need for careful handling of the mold assembly due to the relative fragility of the green sand, as well as undesirable dimensional variations between castings associated with mold cavity and core misalignment and pattern wear. Additionally, green sand molding techniques typically employ core sand compositions which differ from molding sand making reclamation of these components difficult in that they are mixed during the part removal process and thus can cross-contaminate each other. Furthermore, multiple parts are typically cast at one time by using a plurality of part patterns to form several mold cavities within a single flask using a system of common runners. Such an arrangement increases the possibility of a number of parts scrapped due to core mold assembly misalignments and cold-shunting. What is needed is an improved casting apparatus and method to overcome these and other drawbacks.
The present invention disclosed herein addresses traditional shortcomings of green sand molding by employing a variation on the phenolic urethane cold-box system to produce stronger molds and cores of higher dimensional accuracy. Although other core and mold making methods may be embodied within this invention, the cold-box system employs molding sand impregnated with phenolic urethane “no-bake” (hence “cold-box”) binders typically used to form molding cores. One principal advantage of using a phenolic urethane binder is that it can be rapidly catalyzed at room temperature by means of an amine vapor that is blown through the core sand to produce durable cores. Removal of the core from the cast part is made easier by carefully controlling the composition of the phenolic urethane impregnated sand and curing conditions. This invention extends the use of the cold-box system to include forming the mold as well as the core resulting in a sturdy core mold assembly that has superior dimensional stability as well as improved structural integrity that permits more aggressive handling of the mold components as compared to the need to more carefully handle molding assemblies that use a relatively fragile green sand. Furthermore, this approach reduces the likelihood of misalignments in a core mold assembly and improves the finish of the cast part, consequently reducing finishing costs and part scrap rate. Depending on the part geometry, this invention also may reduce the number of needed cores used to produce a cast part. In contrast to multiple-part green sand molding methods, this invention also may be employed to form individual or modular core mold assembly units used to form individual parts. This invention also teaches a method of embodying a plurality of such modular core mold assembly units within a single external flask assembly using a system of gates and runners to produce multiple but separate parts at one pouring, eliminating the possibility of multiple part defects associated with mold misalignment of integrated parts in one core mold assembly and thereby isolating such defects to individual core mold modules and reducing potential part scrap rates.